This application is based on Application No. 117470 filed in Japan on Apr. 16, 2001, the content of which is incorporated hereunto by reference.
The present invention relates to a gallium nitride phosphor usable in, for example, vacuum fluorescent display, field-emission displays (FED) and projection tubes, and relates to a method for producing it. In particular, the invention relates to such a gallium nitride phosphor of good luminescence and to a method for producing it.
Basically having a cathode and an anode opposite to it, vacuum fluorescent display and FED are flat panel displays which are so constituted that the fluorescent film disposed on the side of the anode therein is excited by electron rays to emit light. The acceleration voltage of the electron rays to excite the anode is generally at most 0.2 kV in vacuum fluorescent display, and approximately from 0.1 to 10 kV in FED. The level of this acceleration voltage in these is low, when compared with that in CRT of generally tens kV. Therefore, in general, phosphors capable of being excited by low-energy electron rays are used in vacuum fluorescent display and FED.
Except green-emitting ZnO:Zn phosphors, conventional phosphors that are excited by low-energy electron rays to emit light are phosphor particles coated with indium oxide having the ability to increase the electrical conductivity of the particles. The phosphors of this type include ZnS:Zn (blue), ZnS:Cu,Al (yellow green), ZnS:Au,Al (yellow green), (Zn,Cd)S:Au,Al (green yellow to yellow orange), and (Zn,Cd)S:Ag,Cl (orange to red orange). When excited by electron rays, however, these sulfide phosphors emit sulfide gas and decompose to scatter, and are therefore problematic in that they contaminate oxide filaments and their luminous efficiency often lowers. On the other hand, vacuum fluorescent display and FED are desired to have the function of displaying multi-color images, for which it is necessary to develop phosphors capable of being excited by low-energy electron rays to emit different colors. In that situation, it is desired to develop non-sulfide phosphors for emission of different colors and to make them fit for practical use.
One hopeful example of non-sulfide phosphors heretofore studied in the art is a gallium nitride phosphor. For example, JP-A 41686/1976 discloses a Zn or Cd-doped GaN phosphor. The method for obtaining the GaN phosphor disclosed therein comprises burning gallium oxide in an NH3 gas atmosphere to nitride it. In this case, gallium oxide is nitrided from the surface of its particles, but it is difficult to completely nitride the gallium oxide particles and a non-reacted part remains in the processed particles. Therefore, when the gallium nitride phosphor obtained in the method is excited by low-energy electron rays to emit light, its luminescence is low and the phosphor could not be put into practical use for vacuum fluorescent display, etc. Even when other starting materials than gallium oxide such as gallium sulfide are used in producing gallium nitride, the products still have the same problem.
Given that situation, the present invention is to solve the problems noted above and to provide a gallium nitride phosphor of good luminescence.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
We, the present inventors have assiduously studied to solve the above-mentioned problems. As a result, we have found that a gallium nitride phosphor of good luminescence can be obtained by coating the phosphor particles with a surface-treating compound that contains at least one of P and Sb. On the basis of this finding, we have completed the present invention.
Accordingly, the gallium nitride phosphor of the invention is characterized in that its particles are coated with a surface-treating compound that contains at least one of P and Sb.
Preferably, the total of P and Sb in the surface-treating compound to coat the gallium nitride phosphor is from 0.0001 to 10.0 parts by weight relative to 100 parts by weight of the phosphor. If the amount of the surface-treating compound that coats the phosphor is smaller than 0.0001 parts by weight, the compound will be ineffective; but if larger than 10 parts by weight, the excess compound will interfere with the phosphor excitation and emission and will lower the phosphor luminescence.
The surface-treating compound that contains at least one of P and Sb is a phosphorus compound and an antimony compound, including, for example, phosphates and antimonates. In particular, phosphates realize better luminescence. They include, for example, alkali metal phosphates, alkaline earth metal phosphates, and gallium phosphate. Gallium phosphate is the most effective for improving the luminescence of the phosphor treated with it.
The gallium nitride phosphor that may be processed in the invention includes those of the following general formula:
(Ga,In)N:Y,Z 
in which Y is at least one selected from Be, Zn, Mg, Ca, Sr, Ba, Cd and Hg; and Z is at least one selected from O, S, Se, Te, Pb, C, Si, Ge and Sn.
The gallium nitride phosphor further includes others of the following general formula:
(Ga,In,X)N:Y,Z 
in which X is at least one of B and Al; Y is at least one selected from Be, Zn, Mg, Ca, Sr, Ba, Cd and Hg; and Z is at least one selected from O, S, Se, Te, Pb, C, Si, Ge and Sn.
Basically, any gallium nitride phosphor realizes the effect of the invention. In particular, those of the above-mentioned formulae realize extremely good luminescence.
One method for producing the gallium nitride phosphor of the invention comprises a coating step of bringing a surface-treating liquid that contains at least one of P and Sb into contact with the surface of a gallium nitride phosphor to thereby make the surface of the gallium nitride phosphor coated with the surface-treating compound that contains at least one of P and Sb, and a drying step of drying the phosphor coated with the surface-treating compound that contains at least one of P and Sb.
According to the method for producing the gallium nitride phosphor as above, the surface of the phosphor particles is coated with the surface-treating compound that contains at least one of P and Sb, and the luminescence of the thus-coated gallium nitride phosphor is enhanced. Excitable by low-energy electron rays to emit light of extremely high brightness, the coated gallium nitride phosphor of the invention is expected to be effective in vacuum fluorescent display, etc.
In the coating step, the surface-treating liquid to be used may contain Ga, in addition to at least one of P and Sb.
In the coating step, if desired, the gallium nitride phosphor to be coated may be put into the surface-treating liquid to form a phosphor slurry, then the pH of the phosphor slurry is controlled, and the surface of the phosphor particles may be coated with the surface-treating compound.
In the coating step, the gallium nitride phosphor may be added to the surface-treating liquid to form a phosphor slurry, and the phosphor slurry may be stirred. While stirred, the non-reacted gallium oxide that will remain in the surface of the gallium nitride phosphor particles immediately reacts with P and/or Sb in the surface-treating liquid. The reaction forms a surface-treating compound such as gallium phosphate or gallium antimonate on the surface of the phosphor particles. The phosphor particles are thus coated with the surface-treating compound.
The pH of the phosphor slurry prepared by putting the phosphor into a surface-treating liquid may be controlled to thereby make the phosphor particles coated with the surface-treating compound. In this method, the gallium ions having been released from the surface of the phosphor particles by controlling the pH of the phosphor slurry are reacted with P and/or Sb. Also in this method, the surface-treating compound such as gallium phosphate or gallium antimonate is formed on the surface of the phosphor particles.
The surface-treating liquid that contains at least one of P and Sb may be an aqueous solution of phosphoric acid, antimonic acid, phosphates, hydrogenphosphates, dihydrogenphosphates or antimonates. Aqueous solutions of phosphoric acid or phosphates are more preferred. Phosphoric acid is preferably orthophosphoric acid or pyrophosphoric acid. Phosphates are preferably soluble in water. For example, preferred are alkali metal phosphates such as ammonium phosphate, ammonium pyrophosphate, potassium phosphate, sodium phosphate.
The surface-treating liquid may be an aqueous solution that contains Ga in addition to at least one of P and Sb. To add Ga thereto, gallium chloride, gallium nitrate, gallium sulfate or the like Is added to and dissolved in the surface-treating liquid, or an aqueous solution of such a gallium compound is added to the surface-treating liquid. In the Ga-containing surface-treating liquid, Ga reacts with P and/or Sb therein and to coat phosphor particles.
Another method for producing the gallium nitride phosphor of the invention comprises mixing a surface-treating compound that contains at least one of P and Sb with a gallium nitride phosphor in a solvent to form a phosphor slurry, followed by drying the phosphor slurry to thereby make the phosphor particles coated with the surface-treating compound.
The surface-treating compound to be used in the method is preferably a phosphate or an antimonate, more preferably a phosphate. The phosphate may be soluble in water, such as alkali phosphates; or may be insoluble or hardly soluble in water, such as alkaline earth metal phosphates or gallium phosphate. For the surface-treating compound, gallium phosphate is the best. In case where the surface-treating compound is a water-insoluble or hardly water-soluble phosphate, it is desirable that the phosphate particles have a mean particle size of at most 0.5 xcexcm. If the phosphate particles are larger than the range, a dense phosphate film could not be formed on the phosphor particles, and the surface-treating compound will be ineffective. The solvent may be water or an organic solvent such as methanol.